As those skilled in the art are aware, the threshold voltage (V.sub.T) of an insulated gate semiconductor device is related to the voltage at which the insulated gate semiconductor device turns on. For example, the gate-source voltage (V.sub.GS) of an N-channel insulated gate semiconductor device must exceed V.sub.T to form a channel in the device. Since the voltage at the gate is typically the input voltage of the insulated gate semiconductor device, the input voltage relative to the source voltage must exceed V.sub.T for a channel to be formed. In other words, a significant drain current (I.sub.D) will not flow through the channel region of the insulated gate semiconductor device unless V.sub.GS is greater than V.sub.T. In addition, V.sub.T is important in determining the saturation current of an insulated gate semiconductor device.
An important factor affecting the threshold voltage is the dopant or carrier concentration in the channel region of the insulated gate semiconductor device. V.sub.T can be changed by adding a dopant of either P or N conductivity type to the channel region. For example, when the substrate material of the channel region is doped with a dopant of P conductivity type, V.sub.T can be lowered by adding a dopant of N conductivity type and raised by adding a dopant of P conductivity type. Although modulating V.sub.T by changing the dopant concentration of the channel region is a useful technique, it can be undesirable when the channel region is doped in an uncontrolled fashion. One example of uncontrolled doping is when autodoping from the gate electrode occurs. In this instance, dopant from the gate electrode diffuses through the gate oxide into the channel region. Because the doping is uncontrolled, V.sub.T is not stable.
Accordingly, it would be advantageous to have a method for manufacturing semiconductor devices that is resistant to modulation by autodoping. It would be of further advantage for the method to be compatible with standard semiconductor processing techniques and to be cost efficient.